This invention relates to a new and improved extrusion die assembly and more particularly to such an assembly having a movable mandrel therein that controls the opening in the die head thereof to control the wall thickness of a parison being formed at such opening.
In the processing of thermoplastic material by a die head, it is important to control the wall thickness of the extrudate as it issues from the die head in order to properly control the type of parison being formed, and the subsequent article being formed. Where unusual shaped plastic bottles are being formed from the parison, it is imperative to control the wall thickness such that the thicker material is at that location on the parison which will undergo the greatest amount of stretching. By properly controlling the wall thickness of the parison, one conserves material and enhances the quality of the product. Prior art devices have attempted to control the die head opening relative to the torpedo by manipulating the die head itself; however, this has proved unsatisfactory in design and not responsive to producing a quality product. The instant invention proposes to control the wall thickness of the parison by controlling a linearly adjustable or movable mandrel within the torpedo that varies the opening of the die head. The movement of the mandrel is done internally by means of hydraulic power which simplifies the controls and reduces the number of movable parts thereby improving the response time to a command signal which action improves accuracy and repeatability of the movement. By using the internally movable mandrel the flow channel is not interrupted as with cross head designs. Further with such designs incremental changes in wall thickness can be more accurately controlled. By use of the internally movable mandrel and piston control, rapid disassembly is facilitated of the portions that need cleaning. The sensing device for locating the position of the movable mandrel relative to the die is mounted internally to minimize damage and provide a more accurate feedback. By specifically locating the sensing device close to the apex of the torpedo to which the coolant is directed, a uniform temperature is maintained on the sensing device which extends the service life thereof.